Method of transporting methane or natural gas

ABSTRACT

1,054,149. Liquefaction and re-vaporization of natural gas. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ N.V. Nov. 8, 1965 [Nov. 9, 1964], No. 47244/65. Heading F4P. Liquefaction of natural hydrocarbon gas or revaporization of the gas in the liquid phase is effected by heat exchange with a refrigerant having a freezing point at atmospheric pressure near the boiling point of methane and having a boiling point above or slightly below ambient temperature. Suitable refrigerants include isobutane, isopentane, isohexane and mixtures thereof. After revaporization of liquid methane transported by tanker, the so-cooled refrigerant is fed into the tanker reservoir, transported back to the supply port and used to liquefy natural gas being loaded into the tanker. The so warmed refrigerant may be used as fuel or transported back to the unloading port.

United States Patent 0 ABSTRACT OF THE DISCLOSURE Methane or natural gas is liquefied at a production point for transport in atmospheric pressure containers by heat exchange with a cold liquid refrigerant which sustains its liquid state after heat exchange with the methane or natural gas. The heated liquid refrigerant along with the methane or natural gas is transported to a consumption point in substantially atmospheric pressure containers where the liquefied methane or natural gas is heat exchanged with the liquid refrigerant to heat the methane or natural gas to a gaseous state and cool the liquid refrigerant. The cool liquid refrigerant is then transported back to the production point in substantially atmospheric pressure containers where it is heat exchanged with methane or natural gas as in the beginning.

The invention relates to a method of transporting methane or natural gas in the liquid state and preferably at approximately atmospheric pressure.

It is known to liquefy methane or natural gas at a production point by cooling it deeply. Since the atmospheric boiling point of methane or natural gas is approximately -160 C. the amounts of energy required for the cooling are relatively large. Accordingly, the necessary capital outlay in the cooling plant is also relatively high. It is therefore desirable to reduce the amount of energy required to liquefy methane or natural gas at the production point.

To achieve this object a method of transporting methane or natural gas has been proposed, comprising the following stages:

(a) At a production point methane or natural gas is liquefied by cooling and passed in the liquid state into a reservoir in which a pressure of preferably approxi mately 1 atmosphere is maintained,

(b) The reservoir thus filled is transported to a conr sumption point,

(c) At the consumption point the liquid methane or natural gas is converted to the gaseous state by bringing it into heat exchange with a refrigerant,

(d) The refrigerant, cooled off as a result of the stage described under (c), is passed into a reservoir,

(e) The reservoir filled with the cooled refrigerant as described in stage (d) is transported to production point,

(f) At the production point gaseous methane or na ural gas is cooled according to stage (a) by bringing it into heat exchange with the refrigerant supplied accord ing to stage (c).

It should be noted that by production point is meant the place where the natural gas or methane is liquefied for dispatch, while by consumption point is meant the place where the liquid natural gas or methane is converted, on arrival, to the gaseous state.

It should be further be noted that the above-mentioned reservoir can be arranged on a vehicle or built into a tanker.

It has been proposed to use as refrigerant liquid oxygen,

3,324,670 Patented June 13, 1967 liquid air, liquid nitrogen or liquid ammonia. These substances, which are gaseous at normal temperature and pressure, have, however, drawbacks which make them less suitable for use as refrigerants.

Liquid nitrogen, for example, has the disadvantage of being relatively difficult to obtain, while it also has the drawback of having an atmospheric boiling point of approximately -196 C. In order, therefore, to transport nitrogen in the liquid state at atmospheric pressure in reservoirs from the consumption point to the production point it is necessary to cool the nitrogen to considerably below the atmospheric boiling point of methane or natural gas, for which purpose a relatively large amount of energy is required. Another drawback of nitrogen is that its atmospheric boiling point lies very far below the ambient temperature so that when giving oft" its cold to the methane or natural gas the liquid nitrogen will pass into the gaseous state. Since gaseous nitrogen has relatively few applications, it has to be discharged into the atmosphere, and this must be regarded as a loss.

Liquid oxygen, like nitrogen, has the drawback of being difficult to obtain. Another disadvantage arising from the use of liquid oxygen as refrigerant is that it is not really possible to transport the liquid oxygen from the consumption point to the production point in the same reservoirs as are used for conveying the liquid methane or natural gas from the production point to the consumption point. The reason why this is not really possible is that oxygen can form explosive mixtures with methane or natural gas. Since the atmospheric boiling point of oxygen, like that of nitrogen, is very low, namely 183 C., the use of oxygen as refrigerant has also the same disadvantages as the use of nitrogen as refrigerant. At the production point the liquid oxygen will pass into the gaseous state and it is not always easy to find useful applications for these large quantities of gaseous oxygen. Discharging it into the atmosphere would, of course, mean an economic loss.

The use of liquid air as refrigerant in the above method has approximately the same drawbacks as the use of liquid oxygen as refrigerant.

Nor is ammonia really suited for use as a refrigerant since its atmospheric freezing point lies relatively far above the atmospheric boiling point of methane or natural gas, namely at 77 C. Moreover, ammonia has the disadvantage of having an atmospheric boiling point which lies far below the ambient temperature, namely at 33 C., so that when giving off cold to the methane or natural gas to be liquefied the liquid ammonia easily passes into the gaseous state with the result that difficulties arise at the production point with the storing of the gaseous ammonia. Since gaseous ammonia is poisonous it cannot be discharged into the atmosphere, but must be worked up, for example, into fertilizers.

In order to eliminate the above-mentioned drawbacks it is proposed according to the invention to use a liquid refrigerant having an atmospheric freezing point below or slightly above the atmospheric boiling point of meth ane or natural gas, and having an atmospheric boiling point above or slightly below the ambient temperature.

The atmospheric freezing point of the refrigerant according to the invention is at most approximately 20 C. above the atmospheric boiling point of methane or natural gas, but is preferably lower than this maximum value. If the atmospheric freezing point of the refrigerant were to be higher than said maximum value the refrigerant could not very Well be used since it would then solidify and consequently could no longer be pumped, at a temperature which is considerably higher than the atmospheric boiling point of methane or natural gas. This means that the methane or natural gas could not be cooled tr a suflicient extent with the refrigerant alone, thus making an extra cooling plant necessary.

The atmospheric boiling point of the refrigerant according to the invention is at most approximately 30 C. below the ambient temperature, but is preferably higher than this value.

A refrigerant which is very suitable for use with the method of the invention is isopentane, which has an atmospheric freezing point of 160 C. and an atmospheric boiling point of +28 C. and which is moreover readily available in large quantities at a reasonable price.

Another substance which could be used as refrigerant, but which has less advantageous physical properties than isopentane, is isobutane which has an atmospheric freezing point of 160 C. and an atmospheric boiling point of l2 C. A drawback of this substance is, however, that the atmospheric boiling point is rather on the low side.

Other substances which are in principle suitable are, for example, hexyne with an atmospheric freezing point of -l50 C. and an atmospheric boiling point of +71 0, methyl ether with an atmospheric freezing point of 138 C. and an atmospheric boiling point of 8 C., or 2-methyl pentane with an atmospheric freezing point of -154 C. and an atmospheric boiling point of +60 C. A drawback of methyl ether is, however, that the atmospheric freezing point is rather on the high side, while the atmospheric boiling point is rather on the low side.

Instead of pure substances as refrigerants, use may also be made of mixtures having both an atmospheric freezing point and an atmospheric boiling point within the said ranges. Examples of such mixtures are mixtures of isopentane or isobutane. Further examples are mixtures of isopentane and isohexane or mixtures of isopentane and normal pentane. Particularly suitable is a mixture container 75% isopentane and 25% isohexane which remains a liquid to minus 170 C., that is 10 C. lower than for pure isopentane. Since methane liquefies under atmospheric pressure at minus 160 C. the use of this type of refrigerant makes it possible to install all refrigeration capacity at the gas consumption p'oint. Suitable mixtures may furthermore, for example, also be those which have an eutectic point in the freezing point.

Other mixtures which may be considered for use are, for example, hydrocarbon fractions with a boiling range below approximately 100 C., preferably between 35 C. and 75 C., obtained by distillation of crude petroleum or of a petroleum fraction which has been subjected to a cracking process. These mixtures consist substantially of paraffinic and olefinic hydrocarbons having 5 and 6 carbon atoms.

The refrigerant, having been cooled at the consumption point, is preferably transported to the production point in the tanks which have been used for conveying the liquid methane or natural gas from the production point to the consumption point. This procedure has the advantage that the tanks are invariably maintained at a low temperature so that heating and cooling of the tanks do not occur.

The refrigerant having been heated at the production point, can if desired be used up on the spot, as fuel for example, or if desired, it can be transported to the consumption point to be there brought into heat exchange with an amount of liquid methane or natural gas which has to be converted into the gaseous state.

It is remarked that the refrigerants mentioned may also be used in storage of methane or natural gas for peakshaving purposes. During periods of small demand for natural gas or methane this can be liquefied by passing it in heat exchange with the cold refrigerant, for example, cold isopentane. The methane or natural gas thus liquefied can then be stored in suitable reservoirs and in periods of large demand for natural gas or methane the latter is gasified by passing it in heat-exchange with the warm isopentane. The cooled down isopentane is then stored until a period of small demand for natural gas or methane arrives again. Then the cold isopentane is used for liquefying a quantity of methane or natural gas which is stored until the demand for natural gas or methane rises again.

I claim as my invention:

1. A method of transporting a gas comprising the following stages:

(a) a first quantity of gas is liquefied at a production point by cooling and is passed in the liquid state into a reservoir in which a pressure of at least approximately one atmosphere is maintained;

(b) the reservoir containing the liquid gas is tranported to a consumption point;

(e) at the consumption point the liquid gas is converted to the gaseous state by bringing it into heat exchange with a liquid refrigerant having an atmospheric pressure boiling temperature above or slightly below the ambient temperature and an atmospheric pressure freezing temperature below or slightly above the atmospheric pressure boiling temperature of said gas to further cool said refrigerant within the liquid phase range;

(d) the cooled liquid refrigerant is passed into an insulated reservoir in which a pressure of approximately one atmosphere is maintained;

(e) the insulated reservoir filled with said cooled liquid refrigerant is transferred to the gas production point, said refrigerant being preserved in the liquid phase at approximately one atmosphere pressure by the insulation of said reservoir;

(f) at said gas production point a second quantity of gaseous gas is cooled according to stage (a), at least some of the heat of vaporization of said gas being transferred in heat exchange with said liquid refrigerant.

2. The method as claimed in claim 1, characterized in that said liquid refrigerant, heated as described in stage (f) is passed, in the liquid phase, into a reservoir and carried back to the consumption point.

3. The method as claimed in claim 1, characterized in that said liquid refrigerant, cooled as described in stage (a) is transported from the consumption point to the production point in the same reservoir as has been used to transport the liquid methane from the production point to the consumption point.

4. The method as claimed in claim 1, characterized in that the atmospheric freezing point of the refrigerant is at most approximately 20 degrees above the atmospheric boiling point of liquefied gas.

5. The method as claimed in claim 1, characterized in that the atmospheric boiling point of the refrigerant is at most approximately 30 C. below the ambient temperature.

6. The method as claimed in claim 1, characterized in that the refrigerant is a mixture of liquids, both the atmospheric freezing point and the atmospheric boiling point of the mixture being within the said ranges.

7. The method as claimed in claim 1, characterized in that the refrigerant is isopentane.

8. The method as claimed in claim 1, characterized in that the refrigerant is isobutane.

9. The method as claimed in claim 6, characterized in that the refrigerant is a mixture of isopentane and isohexane.

10. The method as claimed in claim 9, characterized in that the mixture contains 75% isopentane and 25% isohexane.

11. The method as claimed in claim 6, characterized in that the refrigerant is a mixture ofisopentane and normal pentane.

12. The method as described in claim 1, characterized,

in that said liquid refrigerant remains in the liquid state throughout all stages of said method.

13. The method as described in claim 1 wherein said gas is natural gas.

14. The method as described in claim 1 wherein said gas is methane.

15. A method of liquefying a gas selected from the group consisting of methane and natural gas at superatmospheric production pressure by cooling through bringing it into heat exchange with a cooled liquid refrigerant, having an atmospheric freezing point not substantially greater than 20 degree C. above the atmospheric boiling point of said gas, and having an atmospheric boiling point not substantially greater than 30 degrees C. below the ambient temperature.

16. A method of converting liquid methane to the gaseous state by bringing it into heat exchange with a liquid refrigerant which is cooled off thereby, said liquid refrigerant having an atmospheric freezing point not substantially greater than 20 degrees C. above the atmospheric boiling point of methane, and having an atmospheric boiling point not substantially greater than 30 degrees C. below the ambient temperature.

17. The method as claimed in claim 15, characterized in that the refrigerant is isopentane.

18. The method as claimed in claim 15, characterized in that the refrigerant is isobutane.

19. The method as claimed in claim 15, characterized in that the refrigerant is a mixture of liquids, both the atmospheric freezing point and the atmospheric boiling point of the mixture being within the said ranges.

20. The method as claimed in claim 19, characterized in that the refrigerant is a mixture of isopentane and isohexane.

21. The method a claimed in claim 20, characterized in that the mixture contains isopentane and 25% isohexane.

22. The method as claimed in claim 19, characterized in that the refrigerant is a mixture of isopentane and normal pentane.

References Cited UNITED STATES PATENTS 2,682,154 6/ 1954 Wilkinson 6254 2,784,560 3/1957 Johnson 6254 2,799,997 7/1957 Morrison 6254 2,959,020 11/1960 Knapp 62-54 2,975,604 3/1961 McMahon 6255 3,018,632 1/1962 Keith 6252 3,034,309 5/1962 Muck 6255 3,077,082 2/ 1963 Adams et al. 6252 3,108,447 11/1963 Maher et al. 6254 3,195,316 7/1965 Maher et al. 6254 3,246,480 4/1966 Rigby 6248 LLOYD L. KING, Primary Examiner. 

1. A METHOD OF TRANSPORTING A GAS COMPRISING THE FOLLOWING STAGES: (A) A FIRST QUANTITY OF GAS IS LIQUEFIED AT A PRODUCTION POINT BY COOLING AND IS PASSED IN THE LIQUID STATE INTO A RESERVOIR IN WHICH A PRESSURE OF AT LEAST APPROXIMATELY ONE ATMOSPHERE IS MAINTAINED; (B) THE RESERVOIR CONTAINING THE LIQUID GAS IS TRANPORTED TO A CONSUMPTION POINT; (C) AT THE CONSUMPTION POINT THE LIQUID GAS IS CONVERTED TO THE GASEOUS STATE BY BRINGING IT INTO HEAT EXCHANGE WTH A LIQUID REFRIGERANT HAVING AN ATMOSPHERIC PRESSURE BOILING TEMPERATURE ABOVE OR SLIGHTLY BELOW THE AMBIENT TEMPEATURE AND AN ATMOSPHERIC PRESSURE FREEZING TEMPERATURE BELOW OR SLIGHTLY ABOVE THE ATMOSPHERIC PRESSURE BOILING TEMPERATURE OF SAID GAS TO FURTHER COOL SAID REFRIGERANT WITHIN THE LIQUID PHASE RANGE; (D) THE COOLED LIQUID REFRIGERANT IS PASSED INTO AN INSULATED RESERVOIR IN WHICH A PRESSURE OF APPROXIMATELY ONE ATMOSPHERE IS MAINTAINED; (E) THE INSULATED RESERVOIR FILLED WITH SAID COOLED LIQUID REFRIGERANT IS TRANSFERRED TO THE GAS PRODUCTION POINT, SAID REFRIGERANT BEING PRESERVED IN THE LIQUID PHASE AT APPROXIMATELY ONE ATMOSPHERE PRESSURE BY THE INSULATION OF SAID RESERVOIR; (F) AT SAID GAS PRODUCTION POINT A SECOND QUANTITY OF GASEOUS GAS IS COOLED ACCORDING TO STAGE (A), AT LEAST SOME OF THE HEAT OF VAPORIZATION OF SAID GAS BEING TRANSFERRED IN HEAT EXCHANGE WITH SAID LIQUID REFRIGERANT. 